1. Field of the Invention
The present invention relates to an exposure mask that is used in the field of semiconductors, a focus measurement method, and an exposure apparatus.
2. Description of Related Art
Conventionally, a test pattern that is provided on a test reticule is formed on a substrate via a projection optical system, and the optical characteristics of this projection optical system are measured by measuring these formed marks. In recent years, a greater degree of focusing accuracy has been demanded, and a more accurate measurement method is also demanded for the focus positions of projection optical systems.
The technologies that are described in the two publications U.S. Pat. No. 5,300,786 and Japanese Patent No. 3297423 are known as conventional focus measurement methods and focus test patterns.
The focus measurement method described in U.S. Pat. No. 5,300,786 uses what are known as levenson type phase shift masks, and uses a phenomenon in which by exposing an isolated line pattern in a defocused state an image of this isolated line pattern is moved in a lateral direction (i.e., in a direction that is perpendicular to the optical axis of the projection optical system).
FIG. 7 shows an example of a focus measurement pattern that is used when a focus position is measured using this measurement method. When this pattern is exposed, typically a box-in-box mark type of pattern is formed. When this pattern is exposed in the defocused state, then the position of the rectangular pattern (i.e., the box pattern) on the inner side and the position of the rectangular pattern (i.e., the box pattern) on the outer side are moved in opposite directions. By measuring the relative positional relationships of these box patterns, it is possible to measure the amount of defocus at the time of exposure.
However, in the measuring of this box pattern, there are only two patterns being measured in each of two orthogonal directions, and it is recognized that when the measurement is made using image pickup elements of an alignment system arranged inside the exposure apparatus then there is a low level of measurement accuracy. In order to resolve this, the number of measurement patterns is being increased. Specifically, measurement marks are not formed by edges as is the case with box-in-box marks, but by bar-in-bar marks that are made up of a plurality of parallel lines. As a result, it becomes possible to measure a measurement pattern having twice the number of lines as the box-in-box marks.
However, in this measurement method, exposure is actually performed twice, and the measurement pattern is formed by cutting out a portion of a number of diffraction grating patterns. Because of this, this measurement method has the drawback that if there is a large amount of defocus and the position movement exceeds half the pitch between diffraction gratings, then measurement is not possible.
The focus measurement method described in Japanese Patent No. 3297423 makes use of the following phenomenon. Namely, the diffraction efficiencies of positive primary light and negative primary light in a diffraction grating pattern are differentiated from each other (ideally one is set to zero) so that an asymmetrical diffraction grating pattern is formed, and when this asymmetrical diffraction grating pattern is exposed in a defocused state, an image of the asymmetrical diffraction grating pattern moves in a lateral direction.
An example is shown in FIG. 8A of a focus measurement pattern that is used when a focus position is measured in this measurement method. When the reference patterns are the large isolated patterns 21a and 21b, then when an asymmetrical diffraction grating pattern 10 is exposed in a defocused state, the position of the image of the asymmetrical diffraction grating 10 moves, for example, in the direction indicated by the arrow relative to the positions of the images of the isolated patterns 21a and 21b. Moreover, as is shown in FIG. 8B, the reference patterns may be diffraction grating patterns (i.e., 22a and 22b), or, as is shown in FIG. 8C, the reference patterns may be asymmetrical diffraction grating patterns (i.e., 23a and 23b).
However, in these asymmetrical diffraction grating patterns, because one of either positive primary diffracted light and negative primary diffracted light is set to substantially zero, there is a possibility that the exposure amount will be insufficient when the pattern is exposed on a substrate. For example, in the case of a scanning exposure apparatus that performs exposure by relatively scanning a mask and a substrate, the scanning speed must be slowed down and conditions different from normal circuit pattern exposure conditions are used. As a result, there is a possibility that results will be created that are different from the situation at the time of the actual exposure.
The present invention was conceived in view of the above described circumstances, and it is an object thereof to provide a focus test mask that enables a pattern to be exposed in the same conditions as the actual exposure conditions, and that has a test pattern capable of being measured by an image pickup element that is provided inside the exposure apparatus, and to provide a measurement method and exposure apparatus that use this focus test mask.